In that case, you would not want to use a Discount for your free shipping. Instead, you would go into Seller Admin > Cart Shipping Settings, click Next to reach the Shipping Rules screen, and create a Shipping Rule that charges a Flat rate of 0.00 for the United States when Min. Item Total=150.00. You may also need to delete and re-create your other Rule for the US, so it has a Max. Item Total threshold of 149.99.